1. Field of the Invention
The invention pertains to a process for removing heavy metals from soil and paint chips.
2. Description of the Related Art
The removal of heavy metal contaminants from soils represents a major contemporary environmental problem. Heavy metal pollution can leave the affected ground unusable for agricultural or residential purposes, and the metals can eventually leach into the groundwater system and lead to more widespread problems. While a number of soil classification or solidification/stabilization techniques which leave the offending metals in the soil have been developed, only removal of the metals actually solves the problem by removing the cause. Several attempts to remove metals from soils have been reported but none have been completely successful. One such method is described in Draft Report to the EPA, Contract No. 68-03-3255, US EPA, Emergency Response Branch, Edison, N.J., 1986 and CSIRO Aust. Div. Soils Tech. Pap. No. 41, 1979, 1-17 wherein the extracted metals which were bound to clay and humic materials were removed with a strong complexing agent such as EDTA. However, the EDTA remained in the wetted soil causing the treated soil to fail the TCLP test of the EPA for extractable metals. Sci. Total Environ. 1989, 79, 253-270; Geoderma 1979, 5, 197-208; Soil Sci. Soc. Am. J. 1986, 50, 598-601; Can. J. Soil Sci. 1976, 56, 37-42; Can. J. Soil Sci. 1969, 49, 327-334; and Plant Soil 1973, 38, 605-619 teach the use of aqueous acetic acid or ammonium acetate solutions as extractants. This method resulted in only slight leaching of metals from the soil. EP 278,328 (1988); Environ. Prog. 1990, 9, 79-86; EP 377,766 (1990); and Chemiker-Zeitung 1982, 106, 289-292 teach the use of strongly acidic leachant solutions such as HCl. This method leads to substantial (ca. 30%) dissolution of soil components and requires extensive basification of each HCl extract and/or the washed soil. DE 3,703,922 and DE 3,705,519 teach that isolation of the metal values is often impossible due to the metals having been precipitated as sulfides. EP 291,746 and SU 1,444,377 teach that complete separation of the soil and aqueous phases in an extraction process is difficult. J. Indian Chem. Soc., Sect. A 1982, 21A, 444-446; Hydrometallurgy 1987, 17, 215-228; Nippon Kinzoku Gakkaishi, 1978, 42, 1007-1012; Hydrometallurgy, 1985, 14, 171-188; J. Indian Chem. Soc. 1985, 62, 707-709; J. Anal. Chem. USSR, 1983, 38, 630-635; J. Inorg. Nucl. Chem. 1970, 32, 3667-3672; Russ. J. Inorg. Chem. 1960, 5, 906; I. M. M. Bull. 1961, 70, 355 and SU 710,487 teach the use of carboxylic acids such as fatty acids or Versatic.TM. acids for extraction of certain metals in standard liquid ion exchange processes. "The Theory and Practice of Ion Exchange: Proceedings of an International Conference," Cambridge, July 1976, Streat, M. ed., Soc. of Chemical Industry: London, 1976, 38.1-38.7; Trans. Instn. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.), 1974, 83, C101-104; Trans. Instn. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.), 1979, 88, C31-35; "Using Solvent-Impregnated Carbon to Recover Copper from Oxidized Mill Tailings," Rep. of Invest., USDI, Bur. Mines, No. 8966, 1985, pp 7 teach the use of solid-supported ion exchange reagents to remove copper from clarified solutions and from a slurry of oxidized mine tailings. Hydrometallurgy, 1982, 8, 83-94; J. Chem. Tech. Biotechnol. 1981, 31, 345-350; Proc.--Indian Acad. Sci., Chem. Sci. 1988, 100, 359-361; Proc.--Indian Acad. Sci., Chem. Sci. 1988, 100, 455-457 teach the use of LIX.TM. 34, LIX.TM. 622, LIX.TM. 51, LIX.TM. 54, LIX.TM. 70, and Kelex.TM. 100 as extractants for lead from aqueous feeds, but only at more basic pHs than with the carboxylic acids extractants mentioned above. "ISEC '86 Int. Solv. Extract. Conf., Preprints, Vol. II," 1986, 19-26; and Hydrometallurgy 1985, 14, 287-293 teach the use of diethylhexylphosphoric acid as a lead extractant under acidic conditions, but the S-shaped isotherm prohibits reducing the Pb concentration in the aqueous feed down to very low levels. Separation Science 1971, 6, 443-450 teaches the extraction of metals from aqueous solutions by SRS-100, a high molecular weight synthetic carboxylic acid. Various soil removal processes which include the use of mineral acids, bases, surfactants, and sequestering agents are reviewed in International Conf. on New Frontiers for Hazardous Waste Management, September 1987; The Fourth Environ. International Conf. 1983, 856-895; and Environmental Progress 1990, 9, 79-86. EP 402,737 teaches that heavy metals are dissolved out of sludge by strong mineral acid, and that the resulting mineral acid solution containing the heavy metals is treatable with a heavily alkaline solution containing flocculent and foaming agent. EP 278,328 teaches a process of extracting heavy metals from contaminated soils by treating the soils with a number of successive acid extractions in a counter-current fashion and precipitating the heavy metals from the recovered acid solutions. DE 3,742,235 teaches removal of heavy metals from contaminated soils by treating the soils with a 2-40 wt. % EDTA solution having a pH of about 6. U.S. Pat. No. 4,824,576 teaches an improved process for the purification of an impure aqueous solution containing heavy metal ions which comprises passing the impure solution through a bed of activated alumina absorbent. U.S. Pat. No. 4,746,439 teaches a process for the decontamination and removal of at least one of silver, lead, chromium(III), zinc, or nickel ions from aqueous waste streams by contacting the contaminated waste water at a pH of from 4 to 6 with an alkaline earth silicate solid having a surface area in the range of about 0.1-1000 m.sup.2 /g. U.S. Pat. No. 4,883,599 teaches removal of metals from aqueous solutions by passing the solutions through an ion exchange material which consists essentially of sulfhydrated cellulose.
Mercury contamination is a particularly difficult and insidious type of contamination to remediate because of the prevalence and interconvertability of ionic and elemental forms of mercury within a single site by natural weathering action as well as biological redox mechanisms. M. Meltzer, et al. [Pollution Technology Reviews, No. 196, Noyes Data Corp, Park Ridge, N.J., 1990, p. 373] teaches that water soluble and insoluble ionic mercury compounds are bioavailable for reduction to mercury metal by bacterial action, including the highly insoluble mercuric sulfide. The conversion of elemental mercury into water soluble ionic forms is also biologically possible as well as conversion into volatile dimethyl mercury. All forms and compounds of mercury are toxic including elemental mercury. ["The Merck Index," 11.sup.th Ed., Merck & Co., Inc., Rahway, N.J., 1989, p 5805; P. C. Bidstrup, "Toxicity of Mercury and its Compounds," Elsevier, Amsterdam, 1964; L. Magos, Br. Med. Bull. 1975, 31, 241-5] The toxicity of the elemental form can not only be experienced by direct ingestion, but also by inhalation due to the relatively high vapor pressure of mercury, 2.times.10.sup.-3 mm (25.degree. C.). The vapor pressure of mercury alone results in a concentration 200 times higher than the maximum allowed contentration, 0.01 ppm. Long exposure to mercury also produces a cummulative effect. A number of processes have been disclosed which claim to oxidatively dissolve elemental mercury and allow the reclamation of the metal. These processes all have disadvantages. Hot nitric acid solutions are known to oxidatively dissolve mercury while reducing nitrate to nitrogen oxides. [DE 3812986, 1989; DE 3703922, 1988] At room temperature an excess of nitric acid is required and extended periods of time. Under field conditions this will produce substantial quantities of volatile and regulated nitrogen oxides which will require scrubbing before venting to the atmosphere. Additionally, nitrate contamination of groundwater remains a concern because of the large excess of nitric acid required. Hydrogen peroxide is also claimed to oxidize mercury metal to mercuric ions. [USSR 431115, 1974] Catalysis by ferric or iodide ion is also reported. [EP 88-118930, 1988] Hypochlorite in combination with hydrogen peroxide is also claimed. [JP Kokai 63156586, 1988] Hydrogen peroxide suffers the drawback in soil remediation use of being decomposed rapidly and irreversibly with manganese dioxide, a ubiquitous soil constitutent. [EP 88-118930, 1988] This reaction produces useless oxygen gas and water. Peracetic acid at 80.degree. C. has been used to produce mercuric acetate from mercury metal. Hydrogen peroxide in the presence of acetic acid was also successful. [U.S. Pat. No. 2,873,289, 1959]. Hypochlorite oxidation of metallic mercury is known. Control of the pH and chloride ion concentration is required to ensure solubility of the mercuric ion. [U.S. Pat. No. 3,476,552, 1969; Eng. Mining J. 1970, 171, 107-9]. Halogens, including chlorine, bromine, and chlorine with a bromide ion catalyst, are known to dissolve mercury metal and mercuric sulfide. [U.S. Pat. No. 5,013,358, 1991; U.S. Pat. No. 3,424,552, 1969; Chem. Abstr. 1990, 114, 232369c; Chem. Abstr. 1988, 109, 173955n] In soil remediation applications, this requires the use of highly toxic, volatile and corrosive materials in highly populated areas which makes this option less desirable than its use in remote mining locations. Additionally, halogens will react rapidly with organic humic matter in the soil to produce substantial amounts of chlorinated material, including chlorinated phenols. These chlorinated species would present difficulties with regulatory agencies. Cyanide solutions are known to dissolve mercury metal to produce soluble mercuricyanide complexes. The danger of using cyanide solutions in populated areas limits the utility of this approach. In addition, thermal methods of removing mercury from contaminated soil by distilling the metal are known. These suffer the drawback of the high cost of heating soil to approximately 600.degree. C. [DE 3928427, 1991; DE 3706684, 1987; "Treatment Technologies," U.S. EPA, Office of Solid Waste, Government Inst., Inc., 1990, p 17-1]. Methods for treating metallic mercury and leaving it in the soil are also known. The long term acceptability of such practice is unknown. One example, ferric chloride oxidation of finely divided mercury metal, produces a thin layer of mercurous/mercuric chlorides which were reacted with a sulfide salt to produce a mercury particle reportedly coated with a layer of mercuric sulfide. This material could be further stabilized by known solidification techniques. [JP Kokai 81 07697, 1981; DE 3814684, 1989]. Amalgamation of metallic mercury with aluminum or iron deposited onto carbon is reported. The amalgam was claimed to be nonhazardous. [JP Kokai 73 75354, 1973; EP 342898, 1989]. Extraction of mercuric ions from the loaded leachate can be accomplished by a number of processes. [J. Ortega, J. Gutierrez, in "Recovery of Valuable Products from Wastes," Ortega et al. ed.] Recovery of mercury from concentrated solutions is also known by electrochemical reduction as disclosed in U.S. Pat. No. 3,647,958 and D. Bender, F. Riordan, "Metal Bearing Waste Streams, Minimizing, Recycling and Treatment," M. Meltzer, et al., ed., Noyes Data Corp., Pollution Technology Review No. 196, Park Ridge, N.J., 1990, p. 298], and by reduction by iron as disclosed in U.S. Pat. No. 5,013,358; by reduction by sodium borohydride [Morton Thiokiol, Inc., Ven Met Brochure, Ventron Division, 1984], and precipitation with sulfide is known [N. H. Feigenbaum, Ind. Wastes (Chicago), 1977, 23, 324].